Wind power is considered one of the cleanest, most environmentally friendly energy sources presently available, and wind turbines have gained increased attention in this regard. A modern wind turbine typically includes a tower, a generator, a gearbox, a nacelle, and a rotor. The rotor typically includes a rotatable hub having one or more rotor blades attached thereto. A pitch bearing is typically configured operably between the hub and the rotor blade to allow for rotation about a pitch axis. The rotor blades capture kinetic energy of wind using known airfoil principles. The rotor blades transmit the kinetic energy in the form of rotational energy so as to turn a shaft coupling the rotor blades to a gearbox, or if a gearbox is not used, directly to the generator. The generator then converts the mechanical energy to electrical energy that may be deployed to a utility grid.
A power output of the generator increases with wind speed until the wind speed reaches a rated wind speed for the turbine. At and above the rated wind speed, the generator operates at a rated power. The rated power is an output power at which the generator can operate with a level of fatigue or extreme load to turbine components that is predetermined to be acceptable. At wind speeds higher than a certain speed, typically referred to as a “trip limit” or “monitor set point limit,” the wind turbine may implement a control action, such as shutting down or de-rating the wind turbine in order to protect wind turbine components from damage.
Additional instances may also exist in which a wind turbine may need to be de-rated. Such derating is typically achieved by reducing either the torque or speed set points of the wind turbine. For example, if a load monitoring system of the turbine is inoperable (e.g. due to installation, maintenance, repair, and/or replacement), the wind turbine should be de-rated to mitigate loads. Though derating the speed may mitigate loads, such derating may also lead to stall especially in low air density conditions.
Accordingly, systems and methods for operating wind turbines to avoid stall during derating would be desired in the art.